Gas-phase protonation and deprotonation of acrylonitrile derivatives N≡C-CH=CH-X (X=CH3, NH2, PH2, SiH3)

A combined experimental and theoretical study on the gas-phase basicity and acidity of a series of cyanovinyl derivatives is presented. The gas-phase basicities and acidities of (N C-CH CH-X, X = CH3, NH2) were obtained by means of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry techniques. The corresponding calculated values were obtained at the G(3)B(3) level of theory. The effects of exchanging CH3, for SiH3, and NH2 for PH2, were analyzed at the same level of theory. For the neutral molecules, the Z isomer is always the dominant species under standard gas-phase conditions at 298 K. The loss of the proton from the substituent X was found systematically to be much more favorable than deprotonation of the HC=CH linking group. The corresponding isomeric E ion is much more stable than the Z ion, so that only the former should be found in the gas phase. The most significant structural changes upon deprotonation occur for the methyl and amino derivatives because, in both cases, deprotonation of X leads to a significant charge delocalization in the corresponding anion. Protonation takes place systematically at the cyano group, whereby the isomeric E ion is again more stable than the Z ion. Push-pull effects explain the preference of aminoacrylonitrile to be protonated at the cyano group, which also explains the high basicity of this derivative relative to other members of the analyzed series that present rather similar gas-phase basicities, GB approximate to 780 kJ mol(-1), indicating that the different nature of the substituents has only a weak effect on the intrinsic basicity of the cyano group. The cyanovinyl derivatives have a significantly stronger gas-phase acidity than that of the corresponding vinyl compounds CH2=CH-X. This acidity-strengthening effect of the cyano group is attributed to the greater stabilization of the anion with respect to the corresponding neutral compound.